High intensity discharge (HID) lamps are utilized in various lighting applications where large areas have to be illuminated such as for instance in factories, parking lots and sporting fields/arenas. In some such applications, it is desirable to have the light source reach a reasonable level of brightness as quickly as possible particularly if there has been a momentary power outage at a sporting event and the lamps have to be restruck so as to allow resumption of the sporting event. Typically, for a metal halide HID lamp, the time to restart the lamp after it has been momentarily extinguished is on the order of approximately 10 to 15 minutes.
To allow for a rapid restriking of the metal halide HID lamp, a circuit arrangement is typically utilized which can provide a starting or ignition signal having a relatively high voltage at a relatively high current level. Such an starting or ignition signal is needed to initiate the desired ionization condition for the constituents which make up the fill contained within the arc tube of such a metal halide HID lamp. An example of such a metal halide HID lamp and associated starting circuitry can be found in U.S. Pat. No. 4,723,097 which is assigned to the same assignee as the present invention and is herein incorporated by reference. To provide for the lamp ignition, the associated circuitry can employ a spark gap device having relatively high voltage and current carrying capabilities. A common spark plug device is an example of such a spark gap device and has associated therewith, electrodes with ends that are separated by a predetermined length. The problem with such a spark plug device is that over time, the ends of the electrodes erode thereby causing the separation gap therebetween to become larger. As a consequence of the larger separation gap, the voltage potential that must be applied across the electrodes to render the spark plug conductive must continue to increase until the potential becomes such a significant value that it cannot be achieved. Of course, once the voltage potential becomes unattainable, the spark plug device and the hot restrike circuit in which it is disposed, become inoperative.
As a solution to the problem of the eroding ends of a spark plug device, U.S. Pat. No. 4,975,624, assigned to the same assignee as the present invention and herein incorporated by reference, proposes the use of a spark gap device which has two electrodes that are disposed in a parallel side-by-side relationship at a predetermined distance from one another. By this approach, the spark which occurs across the electrodes when a high voltage potential is applied, is distributed along the parallel arranged electrodes rather than being confined to the specific end regions where electrode erosion can occur. As such, the invention of U.S. Pat. No. 4,975,624 has proven advantageous in achieving the 20 to 40 kilovolt high voltage pulses needed to hot start an HID lamp. It has been found however that the use of this type of spark gap device typically operates most efficiently with higher wattage metal halide lamps such as those on the order of about 1500 Watts. When it is necessary to hot restart an HID lamp of a lesser wattage, for instance one requiring a standard ballast having a 600 V input voltage level, it has been found that such spark gap devices do not switch at the same voltage every time. Additionally, because the hot restrike circuit arrangement of the previously discussed U.S. Pat. No. 4,975,624 operates at a high input voltage range of between 2000 and 4000 volts, it is necessary to utilize a large input transformer which adds weight, size and cost to the overall circuit as compared to the input transformer that can be utilized with a low voltage input of around 600 volts.
Therefore, it would be advantageous if a hot starting circuit arrangement could be provided for a lower wattage metal halide lamp where a spark gap device having long life and consistent performance characteristics, would be included.